Npasswd Administration Guide

Npasswd subjects password candidates to a series of guessability tests. The password is accepted only if it passes all of them.

Which tests are used and the order of application is customizable in the configuration file. Some tests are mandatory, and others optional (as noted below).

The password tests are:

• History (optional)
• Lexical (mandatory)
• Local (optional)
• Passwd (mandatory)
• Dictionary (mandatory)

History

Password history discourages too frequent password reuse.

Password candidates are compared to the passwords in the users' history and rejected if found. When a password change is done, the encrypted new password is stored in the history database.

Access to the history database should be restricted. However, should the database fall into hostile hands, the security exposure would be minimal. Passwords in the history got there because npasswd was happy with them, hence they should be hard to guess. See below for details.

The password history database location and storage method can be specified in the configuration file. The history methods available are:

• Plain file.
• DBM database (recommended and default if DBM is available).
Npasswd supports the password history mechanism of Digital UNIX (OSF/1) "enhanced security" mode, and uses it in lieu of the history database.

See also: The password history section of the Npasswd Reference Manual.

Lexical

The lexical check includes:
• Enforcing a minimum length of six characters.
• Checking for non-printable or forbidden characters. Some applications or libraries may not deal properly with whitespace and/or control characters in passwords. Default is to allow non-printable and whitespace characters except common tty control characters (e.g. control-s, control-q, control-z).
• Denying excessive adjacent repeated characters.
• Encouraging a diversity of character classes (mixed case, numbers, punctuation).
• Looking for easily guessed patterns (U.S. Social Security, telephone numbers).
These criteria can be modified via the configuration file.

See also: The password checking section of the Npasswd Reference Manual.

Local

This is the hook for the insertion of site-specific tests. The standard code checks if the candidate is a variation of the system hostname, aliases, or entries in the users' .rhosts.

Passwd

The candidate is examined to see if it is derived from the users' passwd information.

Dictionary

Npasswd checks the candidate against words in various dictionaries. The candidate is rejected if it can be derived from any word in the dictionaries. A variant of the Crack password guessing code is used for this purpose.

See also: The dictionary section of the Npasswd Reference Manual.

More about dictionaries

Crack, the most popular password guessing program, makes extensive use of word collections. Each word is permuted in various ways, encrypted and compared to the target cyphertext. The more words that Crack has available, the more passwords it will guess.

The primary function of npasswd is to frustrate bad guys running Crack. To this end, some of Crack has been incorporated into npasswd.

Dictionaries are a set of hash files optimized for fast lookup. They are constructed from word lists, which are text files with one word per line. The dictionary hash files are created with makedict, which produces files with the suffixes .pwd, .pwi and .hwm. The hash files are probably are not portable between CPU architectures, and take up more disk space their source data.

The dictionary hash files normally live in the dictionary directory (@NPASSWD-DICT@), but can be placed elsewhere and pointed to in the configuration file. Multiple dictionary directories can be given.

To make a dictionary accessible to npasswd, copy the hash files (or make symlinks) into the appropriate directory. The files should be world-readable. Npasswd will reject any dictionary with a world-writable hash file.

You can have numerous dictionaries, each generated from a distinct group of words (the suggested way), or one large dictionary.

The more dictionary words available to npasswd, the fewer passwords it will accept. The passwords it does accept will be harder (but not impossible) to guess. The downside is that users may get more frustrated trying to think up acceptable passwords.

There is more about password checking and dictionaries in the Questions And Answers.

Dictionary management examples

To convert a Crack dictionary:

crack-directory/xdawg crack-dictionary-name (without the '.dwg' extension)

@NPASSWD-UTIL@/makedict -o new-dictionary crack-dictionary-name

To make a new dictionary:

@NPASSWD-UTIL@/makedict -o new-dictionary list-1 ... list-n

To add words to an existing dictionary:

@NPASSWD-UTIL@/makedict -o existing-dictionary -m existing-dictionary list-1 ... list-n

To remove a dictionary:

rm -f dict-name.hwm dict-name.pwd dict-name.pwi

To view the contents of a dictionary:

@NPASSWD-UTIL@/unpacker dict-name | more

More about password history

The password history mechanism controls password reuse. If the history mechanism is active, npasswd fetches the password history for a user and compares the candidate to the active passwords in the history. A password is active if it is younger than the age limit and within the depth limit.

The age and depth limits can be set in the configuration file.

The history mechanism is inactive unless the database file(s) exist. The installation process does not create the database. Use the history_admin utility to create and manage the database.

Security considerations

By default the history database file(s) are created read-write only by owner (typically root). This is done to shield the old passwords from public view, even though npasswd was happy with them so they should be less susceptible to being guessed.

For other programs to access the history database (e.g. checkpassword) they may need to run as root. It is not advisable to install checkpassword as set-uid to root (though it will behave properly). A suggested solution is below.

Special considerations for clusters

If npasswd is being used on a member of a cluster which shares a password file, the password history database must in a directory which the other cluster members mount and can write to, preferably with root access.

An alternative is to make the history database owned by a non-root user (this works around NFS root access issues). In that case, npasswd will attempt to use setreuid(2) to temporarily change its effective uid to the owner of the database to update it.

Here is how to change the ownership of the history database:

#
# If you are creating the history from scratch
#
@NPASSWD-HOME@/history_admin -v load < /dev/null
# The confirmation message includes the history database path
chown new-owner history-file*
#
# If you want checkpassword to access the history database, do
#
cd @NPASSWD-SRC@/src/PasswordCheck
make CHECKPW_MODE=4111 CHECKPW_FLAGS="-o new-owner" checkpassword-install

Password history maintenance

The history database requires periodic maintenance. Old and excess passwords, and entries for users no longer in the password file need removing. Periodically (weekly or monthly), run history_admin from cron. Make this purge a part of what is done upon deleting users from the password file.

History management examples

To create the default database:

history_admin load < /dev/null

Populating alternate DBM database from a file:

history_admin -m dbm -f /tmp/new-history load < test-data

To purge the default database with the default limits:

history_admin -l purge

To ignore the configuration file settings:

history_admin -c /dev/null ...

Files Installed

The following files and directories are installed as part of npasswd. All of these files (except the configuration file) can be relocated by directives in the configuration file.

The path @NPASSWD-HOME@ can be changed by re-running Configure.

Default location	What it is
@NPASSWD-HOME@	Directory where all the parts live
@NPASSWD-XBIN@	Directory where vendor utilities are saved
@NPASSWD-DICT@	Main repository for hashed dictionary files
@NPASSWD-DOCS@	Directory for npasswd documentation
@NPASSWD-UTIL@	Directory for miscellaneous programs
@NPASSWD-HOME@/passwd.conf	The npasswd configuration file
@NPASSWD-HOME@/chfn.help @NPASSWD-HOME@/chfn.motd	Help and message-of-the-day for chfn sub-program
@NPASSWD-HOME@/chsh.help @NPASSWD-HOME@/chsh.motd	Help and message-of-the-day for chsh sub-program
@NPASSWD-HOME@/passwd.motd @NPASSWD-HOME@/passwd.motd	Help and message-of-the-day for passwd sub-program
@NPASSWD-HIST@	Password history database

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Document id @(#) AdminGuide.html 1.9
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Last modified 07/20/98